ParticleSystem reduce Object Count
The solution is to use Particle Instance Modifier to instantiate the grass object, instead of Dupli Object option found in the particle system settings.
https://docs.blender.org/manual/en/latest/modeling/modifiers/simulate/particle_instance.html



Benefits of using the modifier to instantiate an object:


much better viewport performance (especially in 2.79)
no need of pre-rotating the models to use random Z rotation (useful for vegetation)
fast conversion to a single mesh by AltC -> Mesh


How to:


Select your (single) grass model and hit AltG to move it to 0,0,0
Add Particle Instance Modifier to this grass model
In the modifier's options, pick the ground object and it's particle system
The grass should now be instantiated similarly like using Dupli Object (if not, check if your particles are alive, or just tick all of these checkboxes under Show Particles When)
To convert the instantiated grass to single mesh, make sure you have the grass selected and press AltC -> Mesh


There are also some disadvantages though, like using vertex groups for particle size (length) doesn't seem to work (maybe there are some more, but nothing important for me - you will have to find out).